In a magnetic recording device in which a read head sensor is based on a spin valve magnetoresistance (SVMR) or a giant magnetoresistance (GMR) effect, there is a constant drive to increase recording density. One method of accomplishing this objective is to decrease the size of the sensor element in the read head. The sensor is a critical component in which different magnetic states are detected by passing a sense current through the sensor and monitoring a resistance change. A GMR configuration includes two ferromagnetic layers which are separated by a non-magnetic conductive layer in the sensor stack. One of the ferromagnetic layers is a pinned layer wherein the magnetization direction is fixed by exchange coupling with an adjacent antiferromagnetic (AFM) pinning layer, for example. The second ferromagnetic layer is a free layer wherein the magnetization vector can rotate in response to external magnetic fields, and is aligned either parallel or anti-parallel to the magnetic moment in the pinned layer to establish a “0” or “1” memory state. When an external magnetic field is applied by passing the sensor over a recording medium at an air bearing surface (ABS), the free layer magnetic moment may rotate to an opposite direction. Alternatively, in a tunneling magnetoresistive (TMR) sensor, the two ferromagnetic layers are separated by a thin non-magnetic dielectric layer. A sense current is used to detect a resistance value which is lower in a “0” memory state than in a “1” memory state. In a CPP configuration, a sense current is passed through the sensor in a direction perpendicular to planes of the layers in the sensor stack.
Ultra-high density (over 100 Gb/in2) recording requires a highly sensitive read head in which the cross-sectional area of the sensor is typically smaller than 0.1×0.1 microns at the ABS. Current recording head applications are typically based on an abutting junction configuration in which a hard bias layer is formed adjacent to each side of a free layer in a spin valve structure. As the recording density further increases and track width decreases, the junction edge stability becomes more important so that edge demagnification in the free layer needs to be reduced. In other words, horizontal (longitudinal) biasing is necessary so that a single domain magnetization state in the free layer will be stable against all reasonable perturbations while the sensor maintains relatively high signal sensitivity.
Referring to FIG. 1, a schematic drawing of a read-back cross track profile is illustrated which is obtained by scanning the read head across a given data track and plotting the read-back amplitude vs. the off-track distance (distance from track center). The 100% amplitude is the read-back signal when the head is positioned perfectly at track center while uMRW-10% and uMRW-50% are the 10% and 50% micro magnetic read widths that are defined by the width of the cross track profile in FIG. 1 at amplitudes corresponding to 10% and 50% of the track center amplitude. A higher cross track resolution read head requires better uMRW sharpness which means reducing the x-axis distance between −x2 and x2 for uMRW-50% and minimizing the x-axis distance between −x1 and x1 for uMRW-10%. In other words, the slope of line 60 should be more vertical to enable higher resolution. As the device size becomes smaller, it is desirable to achieve higher read-back resolution in the cross track direction. As a result, the read head will have less side reading of data tracks on the sides of the current track to reduce interference at high track densities.
In conventional longitudinal biasing read head designs, hard bias films of high coercivity are abutted against the edges of the sensor and particularly against the sides of the free layer. There may be a thin seed layer between the hard bias layer and free layer. By arranging for the flux flow in the free layer to be equal to the flux flow in the adjacent hard bias layer, the demagnetizing field at the junction edges of the aforementioned layers vanishes because of the absence of magnetic poles at the junction. As the critical dimensions for sensor elements become smaller with higher recording density requirements, the free layer becomes more volatile and more difficult to bias. Traditional biasing schemes using a hard magnet bias have become problematic due to randomly distributed hard magnetic grains within the hard bias layer. Since current technology is unable to provide an improved biasing structure that is capable of stabilizing a sensor in an ultra-high density recording device with high reliability while simultaneously achieving high resolution (uMRW sharpness), a new design for effective longitudinal biasing is needed.